Cyclones of the kind used in the paper industry to cleanse a paper pulp suspension from contaminants and impurities in the form of sand grains, metal particles, chips, splinters and larger metal objects, such as paper clips and paper staples that are liable to be contained by paper pulp produced from wastepaper will typically comprise an elongated cyclone chamber which tapers in a direction towards one end thereof and which is provided at its wider end with a tangentially directed inlet for the suspension to be cleaned and an axially directed outlet for cleaned suspension, the accept, and which further includes an axially directed second outlet for the contaminants or impurities, i.e. the reject.
A cyclone separator of this kind operates in the following manner:
The suspension to be cleaned is fed at high speed into the chamber, through the tangentially directed inlet provided in the upper, wider part of the chamber. The input suspension thereby moves helically or spirally on the inner surface of the wall of the separator, in a direction towards the opposite, narrower end of the chamber, i.e. towards the axially directed second outlet. The heavier particles in the suspension, i.e. the contaminants, endeavour to collect against the wall of the cyclone, while the lighter particles, i.e. the fibres, collect in the centre of the cyclone. The contaminants are moved down into the tapering or narrowing part of the cyclone and exit therefrom through the axially directed second outlet. The inner part of the vortex, on the other hand, turns at the lower end of the tapering part of the cyclone and moves axially in an opposite direction, forming a helical or spiralling vortex, and leaves the cyclone through the upper end thereof in the form of a light, clean fraction called the accept. Thus, when cleansing paper pulp suspensions, the accept will essentially contain fibres of the desired nature.
The cyclone chamber of those cyclone separators known hitherto for cleansing paper pulp suspensions have an inner wall which is either smooth or is provided with helically extending screw grooves which facilitate movement of the coarser and heavier contaminants down towards the bottom outlet of the cyclone, as illustrated for instance in Prior Publication U.S. Pat. No. 3,399,770.
During the coarse of cleaning and separating contaminants from the fibre suspension, an individual particle will move in a circular path around the inner wall of the conical chamber without shifting axially. The particle is held in suspension by buoyancy and entraining forces. The buoyancy forces acting on the particle tend to counteract the entraining forces that endeavour to move the particle axially and thus hold the particle in suspension. The particle thus continuously moves on the same level. The particle is also subjected to a centrifugal force, such that the particle will be held against the inner chamber wall and move around the wall in a closed circular path. Thus, the particle will cut into the surface of the inner wall as it moves therearound. The effect of these particles on the inner chamber wall will gradually wear the wall to a state in which it must be repaired or replaced.
The various cyclone separators described in the afore-said patent specification are intended to eliminate this drawback.
However, this earlier known construction is intended to remove contaminants that are found in typical paper pulp suspensions, such as sand, bark and the like.
The present invention, on the other hand, is intended to remove effectively primarily the kind of contaminants that are found in paper pulp which is produced, e.g., from recycled paper, such as paper clips, staples and other heavier particles. The extraction of such contaminants places completely different requirements on cyclone construction.